The use of seismic techniques to obtain information about subterranean geophysical features is very well-known in the prior art. Such techniques are commonly employed in the exploration for and production of hydrocarbons, e.g., natural gas and oil. The advantages and desirability of accurate characterization of subterranean features are self-evident.
Conventional compressional wave seismic land or marine acquisition techniques involve the use of an appropriate source to generate compressional energy and a set of receivers spread out along or near the Earth's surface to detect any seismic signals due to compressional energy being reflected from subsurface geologic boundaries. These signals are recorded as a function of time and subsequent processing of these signals, i.e. seismic data, is designed to reconstruct an accurate image of the subsurface. In simplistic terms, this conventional process has a compressional wave travelling down into the earth, reflecting from a particular geologic layer (due to impedance contrast), and returning to the receiver as a compressional wave. Data from such a process is referred to herein as "PP" data since compressional waves (P) propagate down from the surface (the first "P") and back up to the surface (the second "P"). In reality, many different types of waves are created in conventional acquisition schemes, and the use of receivers with their sensitive axes oriented vertically (approximately parallel to particle motion for compressional waves), as well as the subsequent processing of the recorded data are designed so that the desired type or types of waves (such as signals representing PP data) is enhanced relative to other types of waves whose signals are considered noise.
On the other hand, so-called shear wave data is conventionally acquired by using a source which introduces particle motion transverse to the direction of wave propagation and then detecting the seismic signal with receivers. Two different types of shear waves (denoted herein as "S") may be acquired: Sh, where the particle motion is perpendicular to, or across, the line from the source to the receiver or geophone; and Sv, where the particle motion is along, or in, the plane defined by the source, reflector, and receiver or geophone. While the characteristics and interpretation of these two types of shear waves may be quite different, both types of acquisition are denoted herein as SS to emphasize the symmetry resulting from the fact that both the downgoing wave (the first "S") and reflected wave (the second "S") are shear waves. Shear seismic data may provide additional information about the properties of the subsurface geologic layers which may be valuable in the exploration for hydrocarbons. See, for example, R. H. Tatham et al.; "V P/V S-A Potential Hydrocarbon Indicator", Geophysics 41, pp. 837-849 (1976).
Those of ordinary skill in the art will be aware that shear waves of the Sv type may also be generated by conversion from a compressional wave transmitted through or reflected from an impedance interface. In this so-called "converted shear" situation, the particle motion of the converted wave is transverse to the direction of wave propagation but in-line with respect to the source-receiver direction. These waves may be seen in conventional PP seismic records but it has been shown that their observation can be enhanced by modifying conventional compressional wave acquisition geophone axes slightly (i.e. placing geophones with their detection axes horizontally in-line rather than vertically). Seismic signals which are predominantly shear-waves may then be detected and may also be recorded. These waves arise from the partitioning of the energy of the compressional wave as it is reflected from an elastic interface. Shear waves of this type are variously referred to as converted waves, or PS waves, and are well known among exploration seismologists. See, for example, Ricker, et al., "Composite Reflections"Geophysics 15, pp. 30-50 (1950); see also U.S. Pat. No. 2,354,548, issued Jul. 25, 1944 to Ricker. When properly interpreted, converted shear wave data has been shown to be capable of providing information about the properties of the subsurface similar to no that provided by SS data.
There are two characteristics of converted waves (PS) which distinguish them from either conventional PP or SS waves. First, the travel path is asymmetric; compressional energy travels downward with a compressional velocity VP (Z), and after reflection travels upward with a shear velocity VS(Z). VP(Z) and VS(Z), (where Z represents the depth) are both generally functions of depth, Z, and VS is normally much less than VP. Second, since the shear velocity is usually much smaller than the compressional velocity in the same material, the velocity distribution of a converted wave (i.e. the velocities experienced by the energy travelling down and back up) is much broader than If the wave had been a pure compressional (PP) or pure shear (SS) wave over its entire path.
As is known by those of ordinary skill in the art, so-called "processed" seismic data is derived from raw seismic data by applying such conventional processing techniques as static correction, amplitude recovery, band-limiting or frequency filtering, stacking, and migration. The processed seismic data may be of either the so-called reflection coefficient data type or the integrated trace data type.
Once processed seismic data has been derived, this data must be correlated with such physical characteristics as reservoir continuity, reservoir thickness, pore fill fluid type (oil, gas, water, etc.), lithologic variation, and pay thickness, to name but a few. This correlation is commonly accomplished using seismic data (two or three dimensional) in conjunction with other inputs, such as well logs. Other ways of making this correlation include, e.g., analysis of surface out-crops and statistical modeling exercises.
Those of ordinary skill in the art will be aware that seismic techniques are traditionally employed to detect and characterize geophysical structures or features deep underground, generally in the subterranean regions where hydrocarbon deposits are likely to be found. On the other hand, seismic techniques are not traditionally employed in the oil industry for the purposes of detecting relatively shallower subterranean geophysical structures.
One type of shallow geophysical feature of particular interest is known as shallow waterflow sand. Shallow, overpressured sands constitute a severe hazard to drilling and facilities development because they tend to flow when penetrated. This causes significant drilling and cementing problems. Shallow flows can lead to washouts resulting in casing wear, buckled casing, and well re-entry problems. In some cases, shallow waterflows can breach the seafloor, resulting in loss of both the individual well and other prospect development sites. Over the years, shallow waterflow occurrences have been reported in various oil and gas fields or prospects. With a few exceptions, waterflow incidents occur at water depths exceeding 1,700feet with an average occurrence in 2,830 feet of water. In recorded cases, waterflow problem sands typically occur from 950 to 2,000 feet but have been reported as deep as 3,500 feet below the sea floor. In any event, for the purposes of the present disclosure, the term "shallow" as applied to subterranean measurements shall refer generally to various depths of up to as much as 3,500 feet or so below the sea floor.
In the Gulf of Mexico, one example of a region prone to shallow waterflow sand problems, the shallow waterflow sands were deposited as continental slope/fan sequences during the Late Pleistocene era. Individual sand-bearing units display slumping zones or debris flows with a chaotic seismic character and, in some cases rotated slump blocks. High sedimentation rates and an impermeable mud or clay seal are thought to be the main factors contributing to overpressure in shallow waterflow sands. These sands occur in a number of depositional subbasins that are generally bounded by diapirs.
The United States Minerals Management Service includes for some regions precautions for possible shallow waterflow in geophysical and geological reviews of plans and applications for permits to drill in leases where potential problems can be identified. Various mitigating approaches have been used including drilling a pilot hole in the shallow section, extra conductor casing strings, the use of "Pressure While Drilling," and other logging tools, and additional geophysical and engineering techniques. Various entities are engaged in programs to upgrade catalogs of known shallow water occurrences, improve the classification of waterflows to allow more meaningful cautionary statements, and continue to work with deepwater operators to prevent the economic loss of moving a well location due to shallow waterflow problem. Waterflow problems have been reported in a variety of depositional basins throughout the world.
Detection of the sources of waterflows seismically is a major challenge. Reprocessed conventional three-dimensional ("3D"), digitally processed high resolution two-dimensional ("2D"), and high resolution 3D short-offset processed seismic data can be used to interpret the depositional environment of waterflow zones. Enhancing the resolution of 3D seismic data is limited by, among other factors, data sampling rate, and by source and receiver depths. High frequencies can be attenuated 1500 feet below seafloor. Enhancing resolution of 3D data helps to define stratigraphy. Most of the abnormally pressured shallow aquifers do not have gas content. They are more likely to be transparent seismically. Amplitude maps or seismic cross sections do not pinpoint the source of potential waterflows although they can in some cases indicate sand prone stratigraphic units. Seismic facies analyses along with well log curves and workstation displays may help to delineate safe areas with no waterflows; often, however, such data is not available. Regional seismic stratigraphic maps of individual basins are presently the preliminary tools in identifying waterflow zones.
As noted above, conventional seismic techniques utilizing compression wave detection have not heretofore been considered effective in the detection of shallow drilling hazards. U.S. Pat. No. 5,555,531 to Booth et al., entitled "Method for Identification of Near-Surface Drilling Hazards, " proposes the use of high-resolution 3-D seismic data to identify the existence of near-surface drilling hazards. However, the techniques proposed by the Booth '531 patent appear to involve modeling only the sea floor from the seismic data, and subsequently identifying "near surface" hazards by visual analysis of the rendered surface. Such techniques would not be entirely effective for the purposes of detecting many shallow hazards, which, as noted above, when "shallow" is interpreted as meaning up to 3,500 feet below the sea floor.